Stiffness and strength degradation of damaged truss core composites

Truss core laminates display stiffness and strength/density ratios superior to those seen in foam cored laminates. However, this superiority is held only for ideal shaped struts. If the truss core is damaged, its performance rapidly decreases towards that of a foam. The present study investigates the stiffness and strength degradation with imposed core deformation/damage. This is done for a pyramidal core structure made by electro-discharge machining from AA5083 alloy. The experiments are compared with finite element predictions. The effect of the strain rate sensitivity is studied by performing the tests at different temperatures and by FE simulations with different material data sets. The results show reasonable agreement between experiments and modeling. The stiffness of a damaged truss core rapidly degrades and reaches the performance levels seen in foams after ≈8% of deformation. The results show that a high strain rate sensitivity significantly influences post-buckling core behavior and is able to decrease the stiffness and strength degradation rate.

Structure-property relationships in nylon 6 nanocomposites based on octaphenyl-, dodecaphenyl-POSS, montmorillonite, and their combinations

Binary and ternary nanocomposites were produced by incorporating, via melt compounding, two types of octa-and dodecaphenyl substituted polyhedral oligomeric silsesquioxanes (POSS), montmorillonite (MMT), and combinations of POSS with MMT into nylon 6. The tensile, flexural, and dynamic thermo-mechanical properties of these materials were characterized and their structure-property relationships discussed. The results show that the losses in ductility and toughness experienced after inclusion of MMT into nylon 6 can be balanced out by co-mixing MMT with the dodecaphenyl- POSS to produce a ternary nanocomposite. This trend however was less pronounced in the ternary MMT/octaphenyl-POSS system. Analysis of the microstructure organization in these materials using XRD and SEM sheds some light on understanding the differences in behavior. Both types of POSS particles mixed alone in nylon 6 were found to be polydisperse (500 nm to a few microns in size) and locally aggregated, yielding materials with similar mechanical performance. The co-mixing of MMT with the octaphenyl- POSS served to break down the POSS crystal aggregates, enhancing their micro-mechanical reinforcing action. On the other hand, the POSS crystals were not affected in the MMT/dodecaphenyl-POSS system, which led to improving their toughening ability.